Such things as a missing perforation will reduce the value of an otherwise good stamp when you sell it. This is especially true of corner perforations. There may be a crease in the paper of the stamp, so small as to go unnoticed unless you examine it closely or use a glass. Be certain, if the stamp is of any great value, that the buyer will use a glass, and the crease will not pass unnoticed. If the perforations are very uneven, it will lower the price. If the colored design of the stamp is unevenly centered within the perforations that is another point against the ultimate selling value. A stamp, normally perforated, having one edge without perforations, is of less value than a perfect copy. These stamps are called straight-edged stamps. Avoid them if at all possible.
Mint stamps without gum are of less value than regular gummed specimens. Usually such a stamp was glued to an album page or in a stock book by humidity. Or it may have become stuck to other stamps, making it necessary to soak them apart, thus loosing the gum. Make no attempt to re-gum a stamp to restore it to good condition. First, you cannot duplicate the gum used by the government. Secondly, you cannot spread it evenly on the stamp. Thirdly, you cannot know the exact thickness to apply the gum. If you lose the gum on a mint stamp, you should just try to accept your loss as philosophically as possible.
The same with parted perforations in blocks of stamps. If you have a block on which some of the perforations have parted, you merely have four stamps. No longer can you value it at the premium price of a block. The best thing to do is to part the stamps, saving an intact pair if possible, and make two separate singles.